This invention relates to a liquid crystal compound and an intermediate thereof.
In a modern, highly information-oriented society, innovation in information display technique as well as innovations of information processing, memory and transmission techniques are inevitable. Liquid crystal display devices have been used as digital displays in many applications, such as hand-held calculators, watches, etc., since they have advantages in that it is possible to drive them at low voltage driving (several volts to several tens volts), they consume a small amount of electric power (several .mu.W/cm.sup.2 to several tens .mu.W/cm.sup.2), they are excellent in saving energy, they can be made thin and light-weight, they produce hardly any eye fatigue due to a non-emissive type, they are very economical, and the like. Further, they have recently been applied as flat displays for animations, such as television, in combination with active matrix driving circuits such as TFT (thin film transistor).
Most of such liquid crystal display devices now practically used are of the twisted nematic (TN) display mode, which has a defect in that their response speed is as slow as several tens msec. due to the revolution in a plane including a molecular axis by static forces generated by respect to application of a voltage. Therefore, liquid crystals excellent in high speed response properties have been desired.
In order to respond to such a demand, R. B. Meyer et al planned and synthesized p-decyloxybenzylidene p'-amino 2-methyl butyl cinnamate (hereinafter referred to as "DOBAMBC") having an asymmetric carbon atom, and found that DOBAMBC shows a chiral smectic C phase and that the phase is a ferroelectric liquid crystalline phase [R. B. Meyer et al; J. de physique 36, L-69 (1975)]. The response properties of ferroelectric liquid crystals with respect to an electric field E depend on precession for maintaining the tilt angle constant by spontaneous polarization Ps. The driving force PsE is remarkably great compared with a method for applying dielectric anisotropy in nematic liquid crystals.
When a ferroelectric liquid crystal is sandwiched between a pair of electrodes having a gap thinner than the helical pitch after subjected to an orientation treatment, there is formed a molecular configuration state corresponding to a smectic C structure wherein the helical structure disappears. When a d.c. electric field is applied in this case, it is found that bistable states are formed depending on the polarity of the electric field. By applying this memory effect, it is not necessary to use active matrix driving, and display by a simple matrix becomes possible, which results in making it possible to economically produce liquid crystal display devices with high speed response. In practice, high speed response is confirmed by sandwiching a ferroelectric liquid crystal in a very thin cell [N. A. Clark, and S. T. Lagerwall: Appl. Phys. Lett., 36, 899 (1980)].
But DOBAMBC has defects in that its operating temperature is high (Sc* phase temperature : 75.degree. to 95.degree. C.) and its, stability is not sufficient because of hydrolysis.
On the other hand, European Patent Publication Nos. 0110299 and 0136725 disclose compounds of the formula: ##STR2## wherein R* is an optically active alkyl group containing an asymmetric carbon atom, said compounds being excellent in stability due to having no azomethine group unlike DOBAMBC. But compounds having a hydrocarbon group as a side chain (which is optically active) generally have a higher liquid crystal phase temperature than those having an ether linkage in the side chain. The compound obtained from lactic acid, as disclosed in European Patent Publication No. 0175591, has an ether linkage outside of an asymmetric carbon atom and is resistant to moisture, but is not sufficiently high in spontaneous polarization since the asymmetric carbon atom is far from an aromatic ring.